1. Field of the Invention
The present invention is related to the field of high pressure fuel injectors for internal combustion engines of the closed nozzle type having a metering and injection plunger assembly that is mounted for reciprocal movement within a variable volume metering and injection chamber that is formed in the body of the fuel injector. More specifically, the invention relates to such fuel injectors, particularly unit fuel injectors, where the metering and injection plunger assembly has a return spring against the force of which the metering and injection plunger is raised by the supply of fuel being metered into the metering and injection chamber.
2. Description of Related Art
Fuel injectors of the initially mentioned type are known. One example of such a fuel injector is shown in U.S. Pat. No. 4,463,901 (which is owned by the assignee of this application), and FIG. 1 of this application represents another example of this type of fuel injector that is produced by the assignee of this application. While each of these closed nozzle fuel injections systems represented state-of-the-art systems at the time at which it was developed, the increasing demands for higher fuel economy and further decreases in emissions have placed addition demands for improvements in such fuel supply systems for internal combustion engines. These demands have led to fuel injectors being required to meet stricter performance characteristics, especially for more precise fuel metering.
However, fuel injectors of the above-mentioned type have a return spring which presses down on the metering and injection plunger, collapsing the metering and injection chamber and forcing the metering and injection plunger into engagement with a plunger seat after injecting of fuel into the engine has been completed. The return spring also has the effect of establishing a preload force which must be overcome before fuel can flow into the metering and injection chamber at the start of the next injection cycle. Additionally, in the collapsed condition of the metering and injection chamber, with the metering and injection plunger in engagement with the top surface of the plunger seat, the pressure of the fuel being supplied is only able to work on a limited area of the bottom face of the metering and injection plunger, i.e., an area equal to the total cross-sectional area of the fuel feed passage(s) by which fuel is able to flow into the metering and injection chamber when the plunger is in engagement with the plunger seat. Also, the fuel being supplied is unable to act at or near the center of the plunger bottom face because a T-shaped drain passage is formed in the metering and injection plunger at its center, the inlet end of this passage being blocked by the plunger seat when the metering and injection chamber has been fully collapsed.
In the FIG. 1 injector, the T-shaped passage 1 is provided in the plunger P to form a path of communication between the metering and injection chamber and a fuel drain passage 3 in the injector body for quickly reducing the pressure within the metering and injection chamber so as to produce a positive and predictable end to the injection event. However, this arrangement leads to the need to precisely control the size of the orifice at the inlet end of fuel drain passage 3 since the orifice sizing is the only means of flow regulation for drain passage 3. Furthermore, this construction creates the situation where, at the time that the fuel supply is commenced, the metering and injection plunger P is exposed to two very different pressures, i.e., the high pressure of the fuel supply, which is delivered via a supply passage 5 formed in a fuel distributing plunger seat 6, and the low pressure of the engine drain flowpath which is in communication with the T-shaped passage 1. This pressure differential causes a bleeding of pressure from the supplied fuel, as well as a tendency for the fuel to find a leakage path to the drain passage 3 via the T-shaped passage 1 or about the plunger P. More specifically, the fuel supply pressure must be high enough to produce initial lifting off of the metering and injection plunger P despite the limited exposure of the fuel to the plunger at peripheral areas thereof and without causing the fuel supply to find a leakage path to the drain passage or about the plunger to the top side thereof (which is also at the low pressure of the engine drain flowpath). As a result, these factors combine to produce a hysteresis effect which affects the ability of the injector to rapidly and precisely meter fuel into the fuel metering and injection chamber. In either case, the occurrence of an initial delay in raising of the plunger or the leaking of supplied fuel to a drain has the effect of detracting from the precise control of the fuel metering and injection process that is important to achievement of high fuel economy and low emissions.
In the case of the noted U.S. Pat. No. 4,463,901, a similar T-shaped drain passage is provided in the metering and injection plunger; however, instead of communicating with a drain passage at the end of the injection stroke to quickly reduce the pressure, the metering and injection chamber is brought into communication with a discharge path that leads the remaining fuel back to the fuel supply passage. While this approach avoids the above-noted hysteresis delay problem during commencement of the fuel supply phase (a problem which would exist in this injector, as well, if its discharge passages led to the engine drain flowpath, as is indicated in the patent to be a less desirable, but possible, alternative), it introduces another problem. That is, in unit fuel injectors of the type in question, the quantity of fuel metered is controlled by the pressure of the fuel supplied as well as the time during which it is supplied (called PT metering), and a common fuel supply rail delivers fuel to a plurality of injectors in a timed sequence. Typically, the pressure of the fuel supplied to the injectors is about, e.g., 10 psi to 100 psi; on the other hand, the pressure of the fuel in the metering and injection chamber, when it is discharged at the end of the injection event, is more than an order of magnitude greater, e.g., 20,000 psi. Thus, by discharging the fuel from the metering and injection chamber back into the fuel supply path, a pressure spike is induced into the pressure-controlled fuel supply, and this pressure spike reduces the preciseness with which the fuel supply quantity can be controlled.